Natural gas liquefaction with controlled b.t.u. content

ABSTRACT

1,141,219. Cold separation of gas mixtures. CONCH INTERNATIONAL METHANE Ltd. 8 Aug., 1967 [17 Aug., 1966], No. 36252/67. Heading F4P. Natural hydrocarbon gas is supplied to a distribution system at a controlled B.T.U. value by liquefying a feed stream 2 at a pressure higher than the distribution pressure in indirect heat exchangers 5, 6, 7 cooled by an external refrigerant, diverting through a line 23 a minor portion of the high pressure L.N.G. leaving exchanger 7, expanding it at a valve 24 into a vessel 27 to produce flash gas and liquid heavy hydrocarbons, expanding at valves 42, 46, 51 the remainder of the L.N.G. into a storage vessel 52 maintained at about atmospheric pressure expanding at a valve 31 the hydrocarbon liquid withdrawn from vessel 27 and adding it to boil-off gas withdrawn by a pump 53 from vessel 52 at a rate controlled by a calorimeter 34a and passing the mixture so formed through a line 47a and a compressor 48 to an outlet line 33 leading to the distribution system. L.N.G. leaving expansion valve 42 is joined by flash gas from chamber 27 and is further cooled in an exchanger 21 traversed by a reverse flow of said boil-off gas and heavy hydrocarbon liquid and also by a reverse flow of a minor portion of L.N.G. withdrawn from the outlet line of exchanger 21 and expanded at a valve 43. The main body of L.N.G. is still further cooled in an exchanger 10 traversed by a reverse flow of a minor portion of L.N.G. leaving expansion valve 46. The two reverse flows of expanded L.N.G. are warmed in exchangers 5, 6, 7 and are fed to the outlet line 33. The external refrigerant for cooling exchanger 5, 6, 7 is circulated in a closed cycle comprising a surge tank 9, expansion valves 11, 21, 22 in series and a three stage compressor to which refrigerant vapour leaving exchangers 5, 6, 7 is fed and which delivers to the tank 9. A portion of liquid refrigerant is withdrawn from exchanger 6 and is used to heat a coil 36 in flash vessel 27; the so-cooled refrigerant being returned to exchanger 7.

CONTENT Oct. 22, 1968 c. o. HUNTREss ETAL NATURAL GAS LIQUEFACTION WITHCONTROLLED BJ; .u.

Filed Aug. 17. 1966 United States Patent* "ice ABSTRACT on THEV nlscllosUn y 'A process for maintaininga gaseous stream supplied to Aadistribution system ata-desired B.tfu. value'in which a high pressurenatural gas stream is liquefied while being maintained at highpressure'. A major portion of the liquefied natural gas is-l reduced inpressurefand stored or passed back inheat exchange'. with the naturalgas being vliquefied thereafter passingl to-*the distribution system. Aminor portion of the liquefied natural gas is fiashed to produce aheavier fraction of liquefied hydrocarbons which arevremoved undercalorimeter control, vre'gasified, 'and added to the distribution-systemat a'rate to maintain the B.t.u. va-lue ofthe gaseous stream supplied tothe distribution at the desired value.

Thisinventionrelates generally to the same problem as is dealt with inthe copending application of Bodle and Young, Ser. No. 282,727, nowPatent No. 3,285,719, as-

signed to the assignee of the: present invention, over which it has theadvantages chieflyV ofincreased ope-rating efficiency and f lowercapital investment'. I3his .lis made possible by theuse of asingleexternal refrigerant inthe placeof .twof or more, 'external`refrigerants,'cornmonly employed, and by the process ofuseparating'outthe 'heavier hydrocarbon constituentsfrornthe feedvigas after it hasbeenl converted to aliquid, and whilek itlris `still iat high pressure,relying mainlypupon a reductionin pressure of theA liquefied vgas toproduce thel` required separation. *Y

if i t Descriptionof the invention A The specific lnature of theinvention aswell as other objects ,and` advantages thereof will clearlyappear from a ,description of a preferred embodiment as shown in theaccompanying dra-wing in'which the single' guljeV is a highly'simplified schematic fiow` chart showingl the principle of theinvention.

Referring to theV drawing,'the feed .gas lis typically supplied in amain `feed gas line v2'at a fai'rlyjhigh pressure,

in line 8 from storage tank 9 tothe interior-ofheat ex- 3,407,052 lk'lhifatentedv Qct. 2`2., 1 968 back in lines 15, 1 6 and 17respectively to refrigerant compressor 18, the vapor from` exchanger7being passed through exchangers 6 andS, andthat from exchanger' beingpassed through exchanger 5','-for furtherinterchange of heat energy inorder to' improve the 'efiiciencyf The pressure and temperaturereductions'in exchangers 6V and 7 are 'successively' achieved; by propersettings of the reduction valves 21"and122 as is well-known. inthe art.

The mainvteed'gasin lin'e"2"emerges'from heatiex hchanger 7 still'atsubstantially itsinitial pressure of'635 pounds, but is now at atemperature of 125 FA minor proportion of this" gas is takenpff in line23, and'is reduced in pressure at-valve'24, under control Vof levelcontrol 26, to produce in heavies lfiash drum 27 vflash-gases consistingofthe lighter components,"which'aretreturried ini-line'v 28,jand also ahe'avierfraction of' liquid hydrocarbons remaining in the'bottom'portion of-the drum,

-which remain in the drum at a pressure of 620` pounds anda"tc'mperatu'reof 92` F.; and are withdrawn-in line 29, -and' suppliedthrough valve 31 and lines 32Jand 547e: to the outlet line 33 of thedistribution system, under control of calorimeter 34a, in an amountsufficient to make up the required B.t.u. content.

Refrigerant from exchanger 6 is also taken in line 34 and heat-exchangedwith the" cold heavier fraction in drum 27, by means of coil 36, andthen returned to heat exchanger 7 through valve 37 and line 40, undercontrol of temperature controller 38 which is in turnV controlled by athermocouple (not shown) located in the kettle portion of the drum 27',but can be overridden by a signal in line 39 from the liquid levelcontroller 41 associated with exchanger '7, in order to ensure that theliquid level 'in 7 remains at the desired value.

from 635 pounds down to approximately 615 pounds,

and is 'still further cooled in exchanger 20 by auto-refrigeration, forwhich purposes a snmall amount of the gas is taken off line 2d throughpressure-reduction valve '43, and returned at 80 pounds pressure and atemperature of 208 F. through line44, which is subsequently extendedthrough the heat exchangers S, 6 and 7 in order to utilize the remainingrefrigeration potential of the gas in this line, -finally emerging fromexchanger 5 at 44a,

still at 8O pounds pressure and a temperature of approximately 15 F.,where it is fed to line 33 of the distribution system as output. Thefeed gas continues in line 2d at a temperature of 200 F. into exchanger10, Where again a minor portion of the gas is taken through all of theabove-described exchangers in series to provide further refrigerationeffect, finally emerging in' line 47a as low pressure flash gas, at17.7-p.s.i.a. and 35 F., and is thereafter compressed by boil-offcompressor 4S to y pounds pressure and fed to line 33 for distribution.

The LNG emerging from exchanger 10 in line 2e at about 17.7 p.s.i.a. and254 F. is reduced to substantially atmospheric pressure in reductionvalve 51v and fed to LNG storage tank 52, which may be any suitablelarge-scale storage facility such as an in-ground storage tank. Theboil-off gas from tank 52 at approximately 15 pounds pressure and 260 F.is compressed by means of any suitable compressor 53 to 18p.s.i.a. and258 F., and is supplied in line'54 to join the gasin line 47, forsubsequent further compression by boil-off compressor 48 to 80 p.s.i.a.,at which pressure it is supplied to thedistribution line 33, aspreviously described.

The refrigerant -vapors in lines 15, 16 and 17 are fed to successivestages of a multistage centrifugal compressor 18, which is anotheradvantage of the presentsystem.

In the past, it has been difficult to utilize centrifugal cornt pressorsfor liquefaction rates as large as those envisaged in a typical systemfor which the invention would be used (5.0 MM s.c.f.d.) because of theirlow inlet volume. However, using a refrigerant such as Freon 13 B-l,exchanger l8 has a vapor pressureof 2.7 p.s.i.a., and with this lowsuction pressure the use of a centrifugal compressor is feasible due tothe high inlet a.c.f.m. (actual cubic feet per minute). The arrangementof injecting the other feed streams at the inner stages increases thea.c.f.m. to each stage, which is desirable in the use of centrifugalcompressors in this type of service. The compressed refrigerant emergesin line 54 at approximately 325 p.s.i.a., and is cooled, preferably bywater cooler 56 to 100 F., after which it is supplied to surge tank 9,from which it is withdrawn on line 8 as previously described.

It will be noted that the above system, in addition to being highlyefficient with respect to power requirements, thus enables the use of asingle compressor, with consequent reduction in cost. Furthermore, theheat exchangers, for a liquefaction plant ranging in size up to 5.0l MMs.c.f.d. can be incorporated in a unitized` construction of exchangersstacked one above `the other, all .within a cold box structure and skidmounted for convenient transportation. It is thus apparent thatl theabove-described system is economical in cost as well as eflicient inoperation. The removal of heavies ,while the gas is at high pressure andprior to subcooling has the further advantage of minimizing'thepossibility of riming problems which these heavies could cause in thesubsequent exchangers 21, 10 which are used for subcooling the gasstream. Furthermore, the external refrigeration system can be easilyderimed when necessary Vby passing warm feed gas through the exchangers5, 6 and 7, and collecting the deriming product in the B.t.u. heaviesvessel 27; alternatively, methanol can be injected into the feed gasstream yfor rime removal and collection in' the same vessel, thusminimizing the time neededv for the liquefaction unit to be out ofservice or bypassed. It will be apparent that the above simplifieddesign lends itself to shop fabrication resulting in reducedfield-costaud direction. When the LNG stored in the tank 52 is needed tosupplement the normal supply provided to the consumer distributionsystem, e.g., during periods of peak demand, it is pumped out of storageand regasified in any known manner, and supplied to the distributionline without any further B.t.u. treatment being required, as it has theproper B.t.u. content by virtue of the above-described process.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of the invention as defined in the appended claims.

We claim:

1. A process for supplying natural gas containing different hydrocarbonconstituents as an outlet gas stream from a n-atural gas liquefactionprocess to a distribution system at a desired B.t.u. value, comprisingthe following steps:

(a) supplying a main stream of natural gas at superatmospheric pressurewell above the normal pressure of the distribution system,

(b) liquefying said main stream of natural gas at high pressure toproduce liquefied natural gas at high pressure,

(c) reducing the pressure of a major portion of said liquefied naturalgas to a low storage pressure and storing it at said low pressure,

(d) removing a minor proportion of said liquefied natural gas at highpressure and reducing its pressure to produce flash gases and a heavierfraction of liquefied hydrocarbons, removing the heavier fraction fromthe separator exclusive of the flashed gases under calorimeter control,and

(e) regasifying said heavier fraction and adding it to the distributionsystem -at a rate to maintain the -,B.t.u..value of the outlet gasstream at a desired value.

2. A process according to claim 1, including the step of returning theflash gases from step (d) of claim 1 to the main stream.

3. A process for supplying natural gas containing different hydrocarbonconstituents as an outlet gas stream from a natural gas liquefactionprocess to a distribution system at a desired B.t.u. value comprising l,

(a) supplying a main stream of natural gas at superatmospheric pressurewell above the normal pressure of the distribution system, A

(b) liquefying the main stream of supply gas at superatmosphericpressure by heat exchange with a single external refrigerant and byauto-refrigerative heat exchange with returning flashed gasesv derivedfrom the supply gas,

(c) reducing the pressure of the thus liquefied natural gas in stages toa low storage pressure, and storing the liquefied natural gas at saidlow pressure,

(d) supplying boil-off gas from said storage, together with flashed gasfrom the pressure reduction stages to said distribution system,

(e) prior to a pressure reduction stage, taking off, from the mainstream of liquefied gas, a minor portion of liquefied gas and reducingits pressure to produce flash gases and a heavier fraction of liquidhydrocarbons, removing the heavier fraction from the separator exclusiveof the flashed gases under calorimeter control, and

(f) regasifying said heavier fraction and adding it to the distributionsystem at -a rate' to maintain the B.t.u. value of the outlet gas streamat adesired value.

4. A process as claimed in claim 3, and the further step of retu'rnin'gthe flash gases from said minor proportion of`liquefied natural g'as tothe main stream.

5. A process as claimed in claim 3, said superatmospheric pressure beingin the order of 635 p.s.i.a., and the single refrigerant being capableof cooling the feed gas at said pressure from F. to 125 F.

6. A process as claimed in claim 3, wherein the heat exchanger with asingle refrigerant is performed in three successive stages ofrefrigeration.

7. A process as claimed in claim 6, wherein the autorefrigeration isperformed in two successive stages following the external refrigeration,by withdrawing a small proportion of liquefied natural gas from the mainstream at each of said two stages, reducing the pressure of saidwithdrawing liquefied natural gas, and passing it in heat exchangerelationship with the main stream in all of the preceding stages.

8. A process for supplying natural gas containing different hydrocarbonconstituents to a distribution system at a desired B.t.u. value,comprising the following steps:

(a) supplying a main stream of natural gas at superatmospheric pressurewell above the normal pressure of the distribution system, v

(b) liquefying said main stream of natural gas at high pressure in threestages of cooling by heat exchange with Freon and by auto-refrigerativeheat exchange with returning flashed gases derived from the natural gasto produce liquid natural gas at high pressure, said Freon being at asuccessively lower temperature and pressure in each successive coolingstage,

(c) removing a minor portion of said liquid natural gas at high pressureand reducing its pressure to produce flash gases and a heavier fractionof liquefied hydrocarbons, while another portion of the liquid naturalgas is further cooled, then expanded and divided into a first andsecondV stream of liquid natural gas,

(d) passing said second stream to storage at low pressure,

gasifying the heavier hydrocarbon fractionl, said combined stream offirst stream, vapor from storage and heavier fraction forming an outletgas which is passed in heat exchange'with said natural gas stream tosupply the auto refrigeration of step (b), said heavier hydrocarbonfraction being added at a rate to maintain the B.t.u. value of theoutlet gas at a desired value,

(g) passing a stream of refrigerant Freon vapor from each stage ofcooling, after the arst, through the preceding stages of cooling in heatexchange relationship therewith without significant change in pressure,each said stream being at the pressure of its respective stage,

(h) compressing said respective refrigerant vapor streams of differentpressures by single multistage 20 centrifugal compression, to an initialoperating stream of natural gas.

References Cited UNITED STATES PATENTS 2,198,098 4/ 1940 Vaughan.2,557,171 6/1951 Bodle et al 62-39 XR 2,940,271 6/ 1960 Jackson 62-23 XR2,960,837 11/1960 Swenson et al. 3,020,723 2/ 1962 De Lury 62-40 XR3,194,025 7/ 1965 Grossmann 62-40 XR 3,271,965 9/ 1966 Maher et al.62,-23 3,285,719 11/1966 Bodle et al. 3,315,477 4/1967 Carr 62-23 NORMANYUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

